Method of coating a magnesium metal article



May 24, 1966 H. K. DE LONG ETAL 3,252,215

METHOD OF COATING A MAGNESIUM METAL ARTICLE Filed Deo. 6. 1961 INVENTORS Herbert .DeL0ng and BY Charles W Glesner ln N A .www

United States Patent O 3,252,215 METHOD F CGATIN G A MAGNESIUM METAL ARTICLE Herbert K. De Long and Charles W. Glesner, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Dec. 6, 1961, Ser. No. 157,592 6 Claims. (Cl. 29-458) This invention relates to a novel method of coating articles of magnesium and magnesium base alloy and composite metal structures containing such articles, and to the improved method of reducing general and bimetallic couple corrosion of magnesium and magnesium base alloy articles.

For the purpose of the specication and claims, both ma-gnesium and magnesium base alloys consisting of at least 75 percent by 4weight of magnesium are referred to hereinafter as magnesium metal.

Attempts made heretofore to protect magnes-ium metal articles from general corrosion and bimetallic couple corrosion under light to moderate conditions, ranging from moist atmospheric conditions to salt splash conditions, have not been entirely successful.

It is, therefore, an object of the invention to provide an improved method of protecting magnesium metal articles from general corrosion.

Another object of the invention is to provide an irnproved method of protecting magnesium metal articles from' bimetallic couple corrosion when the article is embodied in a composite metal structure.

Still another object of the invention is to provide a method which provides corrosion protection to magnesium metal articles at moderate cost, yregardless of the absence of regularity or rectangularity of shape Vof the article.

The invention is predicated on the discovery that on adding at least 3.3 oz. of iine tin dust per ygallon to a metal adherent, water-insoluble, liquid synthetic resin, a novel resin primer is obtained which, on being applied to a magnesium metal article, provides improved general corrosion protection; and when applied to the components of a composite metal structure in Which at least one magnesium metal article is assembled in contact wtih a dissimilar metal article, application of the resin primer being made both before and after assembling the parts, the coating provides improved protection from bimetallic couple corrosion for the magnesium metal article.

The mode of protection -by the resin primer is not entirely understood, but it is believed that on exposure of the magnesium metal part or parts to a corrosive aqueous surface lm,.e.g., aqueous salt solution,'some of the tin dust in the coating dissolves, and on entering the aqueous solution, forms an inhibited solution which has very little tendency to attack magnesium metal and the corrosion tendency is very little promoted even when the solution contacts a bimetallic couple consisting of the magnesium metal article and a dissimilar metal article.

The tin dust which is used is iinely divided powder or dust, at least tine enough to pass a #20 sieve, but preferably 200 mesh (i,e., passing a #.1200 sieve, U.S. Sieve Series) or finer. Such a powder is commonly prepared from tin in the form lknown as beta or white tin having a tetragonal crystalline structure. From 3.3 to 16.5 ounces of such tin dust, or powder, per gallon of resin may be used. Generally, little or no improvement is seen on using more than about 16.5 ounces per gallon of tin dust,'through up to 49.5 ounces per gallon have been used without deleterious effects on the corrosion resistance of the magnesium metal coated thereby.

Liquid or solution form synthetic resins which may be used include any metal adherent, substantially water-insol- 3,252,215 Patented May 24, 1966 uble, alkali resistant resin which m-ay be applied as a liquid lm on a metal surface and which is capable of drying or curing and solidifying on the metal surface as a con-tinuous adherent coating. The corrosion protection provided to a magnesium metal article by any of these resins is improved by the addition of tin dust according to the present invention. Since resins as vinyl acetate, vinyl chloride, vinyl acetate-vinyl chloride copolymers, alkyds and acrylates may be used if relatively mild corrosion conditions' are contemplated. However, better results under most conditions -are obtained on using one orf the epoxy resins of the bis-phenol type, or one of the modifed epoxy resins, as understood in the resin art. The most preferred resins providing superior protection are the butyral resins, e.g., polyvinyl butyral. A suitable clear (unpigmented) modiiied polyvinyl butyral resin is sold commercially as vUnichrome .AP-10.

The resin may be applied with or without solvent dispersion, as desired. Gener-ally, the tin dust remains better dispersed in a composition which is not solvent dispersed. The resin may contain small amounts of modiers, plasticizers-or catalysts, as well understood in the art of coating metals with liquid synthetic'resins.

In carrying out the invention, the magnesium metal article to be coated is cleaned, if necessary, in a conventional manner as by solvent degreasing and acid pick-ling to free the surface of grease, mill scale .and chromates. While chromate cleaners are preferably avoided, articles having residual chromates thereon, or a chromate treated surface, are beneficially coated according to the invention. Generally, somewhat superior results are obtained if the magnesium metal article is dried, as by oven baking at 450 `F. lfor 15-30 minutes, to free the article of moisture and occluded -gases before applying an initial coating of primer. The magnesium metal article is then coated, as by spraying, or roller coating, or dipping, as desired,l whereby a film of the order of 0.5 to 2 mils thick is obtained, and air dried. Or, if the resin primer is adapted to be advantageously cured by heating, the coated article may be briefly air dried and then over baked, if desired.

lf the magnesium metal -article is not to be used in a cornposite structure, it is simply coated one or more additional times, air drying or otherwise curing the coating in an appropriate manner.

However, if the magnesium metal article is to be assembled as part of a composite metal structure in which it contacts a dissimilar metal, i.e., any metal having a different solution potential, and especially a more noble metal such as iron, or copper (as in Monel), the dissimilar metal is also cleaned to degrease it and to free it of mill scale, or loose oxide, and chromates, if present, and coated with a layer of the resin primer of the present invention and the coating dried. Then a composite metal structure whichcomprises at least one magnesium metal article initially coated according to the invention, and the said dissimilar metal article, likewise initially coated according to the present invention, is assembled and the parts mechanically fastened together, as by riveting or bolting, and the composite structure itself is entirely coated with a iilm of the composition of the invention. One or more coats may be used, but generally one suffices.3 The composition is conventionally applied by spraying or dipping. The coating is then cured and dried and the composite structure is ready for use, except that additional colored or pigmented coatings consisting of any suitable resin or paint composition may beapplied, if desired, to give the yarticle or composite structure any desired color nish for eye-pleasing eifect `or for color coding of parts..

FIGURES l and 2 in the accompanying drawing depict articles coated according to the present invention.

FIGURE l is a sectional view of a .structural member 11 such as a panel or plate formed of magnesium metal and provided on all sides with a coating 12 consisting of the resin primer of the present invention.

FIGURE 2 is a .cross-sectional view taken transversely through the `seam of an assembly in which two panels, 21 and 22, of magnesium metal a-re disposed in abutting relationship and held together by being fastened to a structural strip 23 formed of, eg., steel or aluminum. Bolts 24 and nuts 2S ot` steel or aluminum serve as fasteners to hold the assembly together, though other types of fasteners may be used if desired.

Each ofthe structural pieces, as shown, was individually coated with the resin primer of the invention before assembling the pieces. Thus panel 21 is provided with a coating 26. The panel 22 is provided with a coating 27, while the ystrip 23 is provided with a coating 2S. The entire completed assembly is also provided with an enveloping coating 29 of the present resin primer.

In an additional embodiment of the composition of the invention, the tin compound, tributyl tin oxide, is employed in combination with tin dust in the resin primer.

Tributyl tin oxide may be represented by the following semi-structural formula:

n-butyl n-butyl nlbutyl-SIn-O-Slnn-butyl n-butyl n-butyl The use of tributyl tin oxide as the sole tin-containing protective or inhibitive material in a resinous coating composition is described in a copending application, Serial No. 837,780, filed September 3, 1959, now Patent No. 3,034,210.

When tributyltinoxide is used in combination with tin powder in the present resinprimer, the amount of tin powder required for eifective reduction of corrosion is reduced, simplifying the problem of keeping the tin powder dispersed in the resin during coating operations. The use of the combination `of the two forms of tin also provides for superior Icorrosion protection compared to the use of tin powder alone. Generally, 8-12 percent by weight of tributyl tin oxide and 2.5 to 5 ounces per gallon of -200 mesh tin dust, in combination, are suitable concentrations of tin-containing ingredients. The most preferred concentrations are about l percent by Weight of tributyl tin oxide and about 3.3 ounces per gallon of the tin dust.

The resin primer containing the combination of tin forms i-s readily applied to the metal surface in the manner described hereinabove for resin primer containing tin dust only.

Examples To illustrate the composition and process of the invention, 3 x 6 metal coupons formed of Ms" thick magnesium metal sheet (AZ31B alloy) were degreased and pickled in an Iaqueous acid solution containing about 8 v percent of nitric acid and 2 percent of sulfuric acid, then water-rinsed, dried and given an anodizing treatment, known as HAE, according to the military specification (Army Ordnance) MIL-M-45202 (ORD). The specic treatment is classed in this military specication as Type Il, Class A, Grade l. A corresponding number of bare, mild, steel coupons x I5%" x Ms were cleaned in an aqueous alkaline solution and dried. Magnesium metal coupons and steel coupon-s were paired, then spray coated with a polyvinyl butyral lresin primer, known as Unichrome AP-lO, which was obtained pigment-free and to which had been added 200 mesh tin powder or dust. The concentration of tin dust which was admixed with the primer was var-ied from pair to pair. The coatings were allowed to dry overnight. Then, each coupon was pierced three times by drilling 1/4" diameter holes at 11/2 centers along the longitudinal axis of the coupon. Each magnesium metal coupon was then attached by means of mild steel bolts to the corresponding paired steel Coupon having an identical coating. The assembly in each instance was then spray coated with the same type resintin powder mix initially Iapplied to the separate parts.

Each coated assembly was thoroughly air-dried (about six hours) and then subjected to a salt spray test in which a 20 percent aqeuous solution of sodium chloride is continuously sprayed on the assembly during a test period hours in this test) according to ASTM specification Bll7-4i9T. The resin-tin powder ratios and the qualitative results ofV the spray test are listed in the following table. The results are given in terms of the visible extent of the corrosive attack on the magnesium metal at the magnesium metal-steel junction.

By way of comparison, an additional magnesium metalsteel coupon pair was made up, cleaned and coated as above, except that the polyvinyl butyral resin contained no tin dust or powder. The coupons were bolted together and again coated with the clear pigment-free resin, then dried and subjected to the salt spray test referred 3.3 Slight attack along entire juncture. 16.5 Light attack along most of juncture,

33 Do. 49. 5 Do. Severe attack along entire juncture.

much lighter attack than in Test N o. l.

In a second test series, magnesium metal coupons were similarly prepared, cleaned and coated as described in the test above, except that the final preparation of each magnesium metal coupon, Ybefore applying the polyvinyl lbutyral resin primer, consisted of the application of the anodizing treatment known as Dow #17 and described in U.S. Patent No. 2,901,409. The resin applied to one pair contained l0 ounces per gallon of 200 mesh tin dust. Another pair was coated with resin` containing l0 percent by weight of tributyl tin oxide and 3 ounces per gallon of -200 mesh tin dust.

By way of comparison, a third pair was coated with clear polyvinyl -butyral resin, while a fourth pair was coated with polyvinyl butyral resin containing 10 percent lby weight of tributyl tin oxide.

Each of the assemblies -was scribed to as to produce an X-shaped scratch centered on the steel strip and extending part way across the magnesium metal coupon. The scribed assemblies were subjected to 20 percent salt spray according to ASTM designation Bll7-49T for a 10U-hour period.

' Visual inspection of the exposed or testedy pair coated with resin containing tin dust showed very slight attack of the magnesium metal along the scratch marks with some attack adjacent the steel coupon. The pair coated with resin containing both tin dust and tributyl tin oxide exhibited the best corrosion resistance, there being only slight attack adjacent the steel, the corroded area being limited almost entirely `to small areas where the scratch marks crossed the magnesium-steel juncture. The comparison pair coated with resin and tributyl tin oxide showed slightly greater attack than the pair coated with resin and tin dust only. The comparison pair coated with clear resin containing no pigment showed severe attack of the magesium metal around the steel and almost enveloping the entire area around and between the scratch marks on the magnesium metal.

The composition and method of the invention having been thus described, modifications -thereof will be apparent to those skilled in the art and the invention is to be considered limited in scope only by the scope of the hereinafter appended claims.

We claim:

1. The method of coating a magnesium metal article having a clean surface free from grease and mill scale, which comprises: applying at least two coats of a resin primer comprising in admixture a substantially waterinsoluble liquid synthetic resin capable of curing, solidiffying and adhering to a metal surface on being spread thereon as a iilm, and at least 3.3 ounces of tin dust per gallon of resin.

2. The method of coating a magnesium metal article having a clean surface Afree from grease and mill scale, which comprises: applying at least two coats of a resin primer comprising in admixture a substantially Waterinsoluble liq-uid synthetic resin capable of curing, solidifying and adhering to a metal surface on being spread thereon as a lm, at least 2.5 ounces of tin dust per gallon of resin, said tin dust being at least ne enough to pass a #200 sieve, and from 8-12 percent by weight of tributyl tin oxide.

3. The method of coating a magnesium metal article which comprises: cleaning said article whereby the surface thereof is made free of grease and mill scale; and applying at least two coats of a resin primer comprising in adrnixture a substantially water-insoluble liquid synthetic resin capable of curing, solidifying and adhering to a metal surface on being spread thereon as a film, and at least 3.3 ounces of tin dust per gallon of resin, said tin dust being at least fine enough to pass a #200 sieve.

4. The method of coating a magnesium metal article which comprises: cleaning said article whereby the surface thereof is made free of grease and mill scale; and applying at least two coats of a resin primer comprising in admixture a substantially Water-insoluble liquid synthetic resin capable of curing, solidifying and adhering to a metal surface on being spread thereon as a lm, at least 2.5 ounces of tin dust per gallon of resin, said tin dust being at least ine enough to pass a #200 sieve, and from 8-12 percent by weight of tributyl tin oxide.

v5. The method of decreasing bimetallic couple corrosion of a magnesium metal article in contact with a dissimilar metal in a composite structure which conprises: applying to each metal article to be assembled in the structure at least one coating of a resin primer comprising in admixture a substantially water-insoluble liquid synthetic resin capable of curing, solid-ifying and adhering to a metal surface on being spread thereon, and at least 3.3 ounces of tin dust per'gallon of resin, said tin dust being at least fine enough to pass a #200 sieve, and each metal article, prior to the application of said resin primer, being cleaned and free of grease, mill scale and chromates; assembling the composite structure; and then applying at least one additional coat of said resin primer to the entire composite structure.

6. The method of decreasing bimetallic couple corrosion of a magnesium metal article in contact with a dissimilar metal in a composite structure which comprises: applying to each metal article to be assembled in the structure at least one coating of a resin primer comprising in ad-mixture a substantially water-insoluble liquid synthetic resin capable of curing, solidifying and adhering to a metal surface on being spread thereon, and at least 2.5 ounces of tin dust per gallon of resin, said tin dust being at least ne enough to pass a #2010 sieve, and 8-12 percent by weight of tributyl tin oxide, each metal article, prior to the application of resin primer, being cleaned and free of grease, mill yscale and chromates; assembling the 4composite structure; and then applying at least one additional coat of said resin primer to the entire composite structure.

References Cited by the Examiner UNITED STATES PATENTS 2,398,738 4/.1946 Gilbert 117-130 2,427,299 9/1947 Muene 29-460 2,832,753 4/1958 Weinberg et ai. 26o-45.75 2,901,722 s/1959 Amon 260-41 X 2,947,646 8/1960 Devaney er a1. 260-41 2,985,617 5/1961 sayier et a1. 26o- 45.75 3,034,210 5/1962 De Long 29-458 CHARLIE T. MGON, Primary Examiner.

WHITMORE A. WILTZ, Examiner. 

1. THE METHOD OF COATING A MAGNESIUM METAL ARTICLE HAVING A CLEAN SURFACE FREE FROM GREASE AND MILL SCALE, WHICH COMPRISES: APPLYING AT LEAST TWO COATS OF A RESIN PRIMER COMPRISING ADMIXTURE A SUBSTANTIALLY WATERINSOLUBLE LIQUID SYNTHETIC RESIN CAPABLE OF CURING, SOLIDFYING AND ADHERING TO A METAL SURFACE ON BEING SPREAD THEREON AS A FILM, AND AT LEAST 3.3 OUNCES OF TIN DUST PER GALLON OF RESIN.
 5. THE METHOD OF DECREASING BIMETALLIC COUPLE CORROSION OF A MAGNESIUM METAL ARTICLE IN CONTACT WITH A DISSIMILAR METAL IN A COMPOSITE STRUCTUE WHICH CONPRISES; APPLYING TO EACH METAL ARTICLE TO BE ASSEMBLED IN THE STRUCTURE AT LEAST ONE COATING OF A RESIN PRIMER COMPRISING IN ADMIXTURE A SUBSTANTIALLY WATER-INSOLUBLE LIQUID SYNTHETIC RESIN CAPABLE OF CURING, SOLIDIFYING AND ADHERING TO A METAL SURFACE ON BEING SPREAD THERON, AND AT LEAST 3.3 OUNCES OF TIN DUST PER GALLON OF RESIN, SAID TIN DUST BEING AT LEAST FINE ENOUGH TO PASS A #200 SIEVE, AND EACH METAL ARTICLE, PRIOR TO THE APPLICATION OF SAID RESIN PRIMER, BEING CLEANED AND FREE OF GREASE, MILL SCALE AND CHROMATES; ASSEMBLING THE COMPOSITE STRUCTURE; AND THEN APPLYING AT LEAST ONE ADDITIONAL COAT OF SAID RESIN PRIMER TO THE ENTIRE COMPOSITE STRUCTURE. 